A disk drive is a data storage device that stores digital data in substantially concentric tracks on a data storage disk. During disk drive operation, the data storage disk is rotated about an axis while a transducer is used to read data from, or write data to, a target track of the disk. A servo control loop is used to position the transducer above the target track, while the data transfer is taking place. The servo control loop uses servo data read from a surface of the data storage disk as position feedback to maintain the transducer in a substantially centered position above the target track during the data transfer. However, because of positioning errors inherent in the disk drive and external forces, the transducer does not remain perfectly centered during the entire data transfer. Instead, the transducer remains within a positional range about the target track that is dictated by the mechanical properties of the drive.
When a transducer moves off-track during a write operation, there is a chance that the transducer might inadvertently write data on or near an adjacent track. In such situations, the data on the adjacent track may be corrupted. In addition, data that is written off-track by the transducer may be difficult or impossible to read during a subsequent read operation on the present track due to its off-track position.
In an attempt to prevent the writing of data on an adjacent track, disk drive manufacturers have developed off-track thresholds that indicate an off-track transducer position beyond which write operations will be suspended. Off-track thresholds may also be used in conjunction with performing read operations.
In general, two types of situations will cause the off-track threshold to be exceeded, thereby resulting in an off-track error. The first type of situation is due to vibration of the disk drive. The second type of situation is due to defective servo data.
When an off-track error is detected, the disk drive enters an error recovery mode. It is desirable to recover from an error as quickly as possible. If too much time is taken in attempting to recover from an error, a time-out error may be received from a host computer associated with the disk drive, which may cause the host computer to lock-up. In addition, if too much time is taken in attempting to recover from an error, disk drive performance requirements may not be met.
There are different types of ways for recovering from off-track errors due to vibration as opposed to recovering from off-track errors due to defective servo data. However, there has yet to be a technique which can distinguish between these two types of off-track errors.
Accordingly, it would be desirable to develop an error recovery method and apparatus that distinguishes between off-track errors due to vibration and off-track errors due to defective servo data, so that appropriate corrective action with respect to each type of error may be taken.